1. Field of the Invention
The present disclosure relates to a sensor, in particular a resonant sensor which may be arranged to measure the pressure of a fluid.
2. Description of Related Art
Resonant sensors have a diaphragm, typically manufactured from silicon, which is exposed to a fluid, the pressure of which is to be measured. Silicon is typically used for the diaphragm as it has a very repeatable elastic behaviour. Variations in pressure of the fluid exposed to the diaphragm cause the diaphragm to flex or deform in accordance with the pressure of the fluid. Resonant sensors are described, for example, in U.S. Pat. Nos. 5,101,664, 5,969,257 and 6,504,864.
FIGS. 1 and 2 illustrate a known resonant pressure sensor 1 comprising a resonator 2, in this example comprising two beams 3 suspended between two pedestals or mesas 4 which are an integral part of a diaphragm 5 to be exposed to a fluid. The resonator 2 is excited to make it vibrate. The flexing of the diaphragm 5 by the pressure applied by the fluid stretches the resonator 2 as shown in exaggerated form for clarity by FIG. 3, altering the frequency at which the resonator vibrates. By measuring the resultant frequency of the stretched resonator, the pressure of the fluid acting against the diaphragm may be inferred.
FIG. 4 is a top view of the resonator showing the vibration of the beams 3 in dashed lines again in exaggerated form for clarity. The beams 3 may be driven by suitable actuators 6, shown schematically in FIG. 4, typically via an AC driving signal. The resonator 2 typically has two beams 3 to make the system symmetrical. However, as shown in FIG. 4, the tops of the mesas 4 are subjected to moments 7 due to the movement of the beams 3 which puts the mesas 4 under stress resulting in them being pushed together due to a net reaction force as shown by dashed lines in FIG. 5. The movement of the mesas 4 towards each other flexes the diaphragm 5 and puts energy into the fluid to be measured reducing the precision of the measurements. Furthermore, the distortion of the diaphragm 5 caused by the moments 7 to which the mesas 4 are subjected also causes distortion of the beams 3 degrading the measurement of the beam position, further reducing the precision of measurements. This reduction in the precision of measurements is of particular concern for low pressure sensors as the distortion of the diaphragm 5 caused by the moments 7 results in a significant transfer of energy from the resonator to the fluid being measured, resulting in a low resonator quality factor leading to reduced sensor accuracy.